1. Field of the Invention
The present invention relates to electronic photography color image forming apparatuses, such as a laser printer, a copier and a facsimile, which include a plurality of photosensitive members.
2. Description of the Related Art
In an image forming apparatus that overlaps multi-color toner images with each other to form a color image, it is important that each color is correctly printed on a prescribed position on a printed matter, i.e., misregistration does not occur in view of quality of products. Hereinafter, “misregistration” is defined as misregistration regarding colors. There may be various factors of misregistration. Among major factors of adverse effects, there is a factor of variation in laser irradiation position on a photosensitive member caused according to thermal deformation of an optical unit. Typically, the optical unit has a configuration that causes a rotating polygon mirror to reflect a laser beam emitted from a light source for scanning. While the laser beam reaches from the light source to a photosensitive member, the beam is reflected by mirrors several times and the traveling direction thereof is changed, and a spot and a scanning width are adjusted via lenses. These elements defining the optical path of the laser beam are fixed to a frame configuring an optical unit. Rise in temperature due to operation of the image forming apparatus thermally deforms the frame, and thus changes the orientations of these elements, thereby affecting the direction of the optical path of the laser beam. Variation in direction of the optical path is increased in proportion to the optical path length reaching the photosensitive member. Accordingly, even if frame deformation is significantly small, variation in laser irradiation position appears. Such variation in laser irradiation position according to a phenomenon of rise in temperature is called a thermal shift of the laser irradiation position. Rise in temperature in an image forming apparatus and rise in temperature of an optical unit due to heating of a motor driving a rotating polygon mirror (self-temperature rise) have been recognized as factors varying the laser irradiation position.
There are following methods for correcting misregistration caused by these factors. For instance, pattern image for matching laser irradiation timing of each color is formed as a toner image on an intermediate transfer medium, and the image is read by a sensor. Accordingly, this method detects the amount of misregistration between a reference color and an object color and corrects an image-writing position. However, this method takes required time for calibration to form a pattern image. A method to address this problem provides a temperature sensor, a misregistration correction section that estimates variation in laser irradiation position based on an output of the temperature sensor to correct laser irradiation timing, and corrects the misregistration without forming a pattern image. Typical configurations are as follows. For instance, one method detects temperature of an optical unit itself by a temperature sensor, and corrects a laser irradiation position by a correction control device (e.g., see Japanese Patent Application Laid-Open No. 2000-218860). Another method detects temperature in an apparatus by a temperature sensor, and corrects a laser irradiation position by a correction control device based on a detection result (e.g., see Japanese Patent Applications Laid-Open No. 2003-207976 and No. 2005-234099). These methods are based on a technical idea that measures temperature at a site affecting a thermal shift, and corrects the laser irradiation position according to variation in temperature. These methods are applicable to cases where variation in temperature and misregistration tendency can be approximated at one-to-one correspondence relationship.
However, in actual products, one-to-one correspondence relationship is not necessarily found in relationships between variation in temperature and tendency of variation in color misregistration. Misregistration correction by estimating variation in laser irradiation position based on a result of measurement of temperature is inapplicable to these cases. More specifically, an example thereof is a case where, even though temperature at a site affecting a thermal shift is increasing, the direction of misregistration variation is reversed in the process. In such a case, it can be considered that relative temperature balance at multiple points complexly deforms an optical unit, which, in turn, varies laser irradiation position. However, it is difficult to identify these points.
For instance, in an image forming apparatus having a configuration where optical units capable of scanning only one laser beam are dedicatedly provided for respective colors, variation in temperature and variation in laser irradiation position can be easily approximated at one-to-one relationship. The optical units for the respective colors have the same configuration. Accordingly, thermal shifts of laser irradiation positions have similar tendencies among all the colors, and a relative difference between colors can be easily found. However, in an image forming apparatus having a configuration where one optical unit can scan multiple laser beams for realizing reduction in size and cost of the image forming apparatus, the optical unit has a complicated structure. For instance, the numbers and shapes of mirrors and lenses where laser beams pass in processes from light sources to the surfaces of photosensitive members are different according to colors. Furthermore, a site and a peripheral structure where these optical elements are fixed to the optical unit are different. According to these causes, variations in laser irradiation position due to variations in temperature sometimes have different tendency among the colors. In an aspect, according to increase in unit packaging density in the apparatus due to reduction in size, thermal effects on the optical unit affected from the periphery become complicated. Owing to the effects of these factors, it is difficult to find correlation between tendency of variation in misregistration and variation in temperature around the optical unit. That is, there is a possibility that variation in laser irradiation position according to variation in temperature cannot be estimated.
To address the problems, a method can be considered that forms a pattern image of a toner image on an intermediate transfer medium between pages in continuous prints, detects the amount of misregistration between a reference color and an object color by reading the image, and corrects the image-writing position. However, if a multi-color toner image is formed between pages, a sufficient number of pattern images cannot be accommodated in prescribed page intervals. There is a method that widens page intervals for forming multiple (multi-color) toner images between pages and performs calibration. This method unfortunately decreases throughput.